All factory libraries are overengineered.

factories.rb

require "ffaker"

class ActiveRecord::Base
  def self.define_factory(name=nil, parent=nil, &block)
    name = [name, "factory"].compact.join("_")
    scope name, lambda { |overrides={}|
      parent ||= scoped
      attributes = instance_eval(&block).merge(overrides)
      parent.where(attributes)
    }
  end
end

User.class_eval do
  define_factory {{
    first_name:            Faker::Name.first_name,
    last_name:             Faker::Name.last_name,
    email:                 Faker::Internet.email,
    password:              "monkey",
    password_confirmation: "monkey"
  }}
end

Comment.class_eval do
  define_factory {{
    title: Faker::Lorem.sentence,
    body:  Faker::Lorem.paragraph
  }}
end

# In order to support associations you *have* to be explicit.
# Yeah, it's not as magic as FG or Machinist, but it works fine.
#
# And I haven't worked in project with such a complex dependency graph
# that requires you to write a lot of associations by hand (maybe it's
# just me.)
#
# Finally, this is WAY faster than any other factory lib out there,
# except for database fixtures. Well, I haven't benchmarked it, but
# I'm pretty sure it is :P
#
# Go ahead and benchmark it if you want, I'd like to see the results.
comment = Comment.factory.create(title: "My Comment") do |c|
  c.commenter = User.factory.create
end